Four-point probe pin devices are used to measure sheet resistance for conductive layers, for example, in semi-conductor applications. Pins for known four-point probe pin devices are isolated from each other by drilled holes with separate bearings or by micro positioning controls and an air gap, commonly called micromanipulators. U.S. Pat. No. 4,383,217 teaches four pins disposed in drilled holes in a retaining strip. The separate bearing method limits the minimum gap between pins due to the pin diameter, bearing thickness, and hole separation distance. The micromanipulator arm approach is expensive and does not allow for the manufacturing of a complete probe head assembly with multiple contacts. Commonly owned U.S. Pat. No. 6,815,959 teaches four pins in a probe pin device with spacers made of separate sheets of insulating material disposed between the pins. The spacers make assembly difficult and the thickness of the spacers acts to limit the extent to which the gap between pins can be minimized. Known configurations also prevent or make difficult replacement of individual pins in a probe pin device.
It is known to use springs to urge a four-point probe pin device toward and into a conductive layer for purposes of measuring sheet resistance. However, unless the spring is exactly centered on the appropriate portion or portions of the probe pin device, force from the spring can skew the probe pin device out of a desired orientation with respect to the conductive layer. Also, the useful lifetime of springs is limited, typically to between 100 to 1,000 touchdowns. Further, to vary spring pressure either additional components such as adjustment screws are needed, or the spring must be removed and replaced with another spring having the desired characteristics.